As is known in the art, a network includes a plurality of processing sites generally referred to as stations or nodes connected by one or more physical and/or logical connections. When the connections establish transmission of a signal in one direction between the nodes, the connections are generally referred to as links. Each node typically performs a switching function and one or more additional functions.
The nodes may be coupled together in a variety of different network structures typically referred to as network topologies. For example, network nodes made be coupled in a circular structure, generally referred to as a ring topology. Other topologies such as star topologies and tree topologies are also known.
The transmission of a signal from a first or source node to a second or destination node may involve the transmission of the signal through a plurality of intermediate links and nodes coupled between the source node and the destination node. Such a succession of links and nodes between a source node and a destination node is referred to as a path.
When a link or node in a path fails, communication between a source node and a destination node in that path is disrupted. Thus, to continue communications between the source and destination nodes, an alternate path must be found and the signal being transmitted from the source node to the destination is routed through the alternate path.
A self-healing network refers to a network which automatically restores connections among nodes in the event of a link or node failure in a path from a source node to a destination node. There is a growing trend and reliance on such networks owing to increasing reliance on and use of high-speed communication networks and the requirement that these communication networks be robust in the case of certain failures. Self-healing networks are central not only to applications where failures occur very often, such as military networks under attack, but also in public networks where failures, albeit rare, can be extremely disruptive. Self-healing networks can also be used in power transmission grids to aid in the distribution of power signals in a power network. Thus self healing networks have use in a wide variety of applications including but not limited to communications networks and power systems networks.
Self-healing networks typically detect and report a failure, establish and connect a restoration path and then return the network to normal communications. Such self-healing characteristics are incorporated, for example, in the Synchronous Optical Network (SONET) protocols.
A network may be represented as a graph which includes nodes representing the network nodes and edges representing bi-directional connections between the nodes. For a network to be capable of having a self-healing feature which leaves all nodes mutually connected even after the failure of a node and/or an edge, a graph representing the network must be either node or edge redundant. A node redundant graph is a graph in which the nodes remain mutually connected even after the elimination of any node in the network. Similarly, an edge redundant graph is a graph in which the nodes remain mutually connected even after elimination of any edge in the network. Node or edge redundancy ensures that all nodes remain connected even after the failure of a node or edge, respectively. One problem, however, is that not all self-healing techniques are guaranteed to work over an arbitrary redundant network (i.e. a network having some portions which are node redundant and other portions which are edge redundant). For instance, a technique which performs self-healing locally may not be able to use the redundancy afforded by a more distant part of the network. Node or edge redundancy is thus the minimum topological requirement for a network to perform self-healing which allows all remaining nodes to remain mutually connected after failure of a node or edge.
Self-healing networks can be classified according to the following three criteria: (1) the use of line (or link) rerouting versus path (or end-to-end) rerouting, (2) the use of centralized versus distributed schemes and (3) the use of precomputed versus dynamically computed routes. The criterion of link rerouting versus path rerouting maps to the criterion, common in optimization, of local versus global. The different criteria are not in practice, selected independently of each other for self-healing networks. Because path rerouting is less local in nature than link rerouting, path rerouting schemes are not usually distributed.
Furthermore, since path rerouting tends to require a relatively large amount of computation and covers a relatively large span of a network compared with link rerouting, path rerouting is typically implemented using precomputed routes. Link rerouting, which usually considers only a few hops away from the rerouted link in the network, is better suited to dynamically computed routes and thus may be done in a distributed fashion.
For example, in a bi-directional self-healing ring (SHR), after a link failure, traffic that was previously carried along one link is carried around the rest of the ring on a second link using a technique generally referred to as loopback. Thus, in this particular case, path rerouting can be used.
One problem with this approach, however, is that a limitation may exist with respect to the number of nodes which may be traversed to replace the failed link. If there is a hop limit, for example, link rerouting may not be possible in a network having a ring topology.
One of the most common ways in SONET to restore rapidly network functionality is to combine SHRs and diversity protection (DP), using add-drop multiplexers (ADMs), for automatic protection switching. Systems using One-to-n (1:n) DP have one back up fiber for n fibers. SHRs perform loopback, which may be regarded as a special case of APS. SHR architectures may be classified into unidirectional rings, in which the duplex channel travels over a different path than the forward channel, and bi-directional rings where the forward channel and the duplex channel travel the same path. Bi-directional rings typically include two or four fibers. Using mechanical ADMs, the restoration time is typically about 50 milliseconds (ms) while path switching typically requires less than 20 ms and loopback switching typically require under 80 ms. DP or SHR architectures typically require about 10 ms to detect and 50 ms to complete the switch.
One problem with the DP and SHRs approaches is that they require built-in excess capacity to handle failures. Moreover such systems may be difficult to upgrade as more nodes are added to the network. A system utilizing one-to-one (1:1) DP requires the most spare capacity, since the spare capacity is the same as the capacity used during normal operation. SHRs may require as much spare capacity as DP, depending upon network traffic patterns.
Furthermore, placing constraints on possible network topology may result in increased network cost. For example, a ring topology may be used in a particular application to implement APS. The ring topology, however, may not be the most cost effective topology for the application. This results in a relatively expensive network.
It would, therefore, be desirable to provide a system which allows APS over any arbitrary network topology. Such a system can be used with any existing topology, allows relatively inexpensive expansion of existing networks regardless of topology and allows construction of new and relatively inexpensive networks.
Another approach to self-healing which is faster than the above approaches is achieved by using optical switches such as acousto-optical switches. Optical switches allow switching to a backup fiber, in a time delay which is in the order of micro-seconds (.mu.s). For four-fiber bi-directional rings, optical switches and amplifiers have been proposed to act as ADMs. Optical wavelength division multiplex (WDM) ADMs have been proposed to add/drop channels within a fiber rather than add/drop whole fibers. Such WDM ADMs are useful to overcome the limitation in capacity of traditional SONET ADMs. Many optical WDM ADMs have been proposed and demonstrated. Being able to drop channels rather than whole fibers also gives more flexibility in operating SHRs. WDM survivable rings using a single switch central office have been proposed.
Still another class of self-healing networks which reduces the amount of spare capacity needed, involves utilizing real-time spare capacity in either a distributed or a centralized dynamic restoration system. Thus, real-time excess capacity is used to transmit in case of a failure. This class of self-healing schemes commonly relies on digital cross-connect systems (DCSs) which provide greater flexibility than ADMs. However, such schemes utilize real-time spare capacity at the expense of relatively complex and computationally intensive software processing and thus greater time delays.
Yet other approaches attempt to perform optimal routing in centralized circuit switched networks with the goal of preserving spare capacity. While centralized restoration offers improvement in terms of spare capacity utilization over distributed restoration, it requires real-time computation having a complexity which increases with increasing network size. Thus, one problem with a centralized restoration approach is that it is relatively slow, with restoration times reaching minutes.
To overcome the relatively slow restoration times which occur in centralized restoration schemes, dynamic restoration schemes utilizing distributed control or a hybrid of distributed and centralized control are used. The greatest improvements in speed are provided by distributed schemes. A variety of different distributed schemes have been proposed, each of which rely on some variation of a sender-chooser scheme where a sender floods a network with help messages and a chooser selects a restoration path based on reception of those help messages. Other schemes involve a single sender-chooser pair, one sender and multiple choosers, two senders and two choosers, etc. . . . Thus, suffice it to say that a variety of different self-healing schemes each having different drawbacks and limitations have been used.
In addition to self-healing features another feature of increasing importance in networks is that of a multicasting operation. Multicasting refers to transmission of a signal from a source node to a predetermined set of nodes in a network. When the set of nodes corresponds to all of the nodes in the network, the operation is referred to as a broadcast operation. It is desirable to provide networks having a multicasting feature since in many types of networks, it is relatively simple for a node to receive a signal and replicate it for transmission to several other nodes. The need for such an ability tends to occur relatively often in relatively high-bandwidth networks.
For instance, a server may transmit a single video signal to several receivers or a file server may simultaneously back up data stored thereon to several geographically separate locations. Multicasting is relatively easy to perform when the network nodes are connected in a tree topology. In networks which allow duplication of signals at nodes, a link need only carry at most a single copy of a multicast signal. All-optical nodes for instance, allow simple replication by splitting a first received optical signal into a pair of optical signals and thereafter amplifying the pair of optical signals. The SONET standard, for example, includes a multicasting standard.
If a network is provided having a tree topology and an edge failure occurs in a tree originating at some source node S, all the nodes in the tree which were connected to the source node S that are not downstream of that failure are still connected by the same tree. The nodes of the tree which are downstream of the failure, however, are no longer connected owing to the edge failure. Complete functionality is restored to the network by re-connecting to the source node S the nodes that were downstream of the failure.
It would, therefore, be desirable to provide a network having a redundant tree topology in which such re-connection is possible for both link and node failures. It would also be desirable to provide a means for constructing a tree topology on any node or edge redundant network so that failure of a node or edge does not disconnect any node from the network.